Ceramic slip casting is a known process by which ceramic powders can be used to produce a desired net shape cast ceramic article. In comparison to other ceramic forming processes (e.g., dry pressing, injection molding and high-energy impacting), intricate net shape articles can be formed by the slip casting process at relatively low production costs. The slip casting process generally involves forming a stable aqueous dispersion of ceramic powders, with the aqueous component most often being water. The aqueous dispersion, referred to as a slip, is then poured into the cavity of a porous mold. The liquid component of the slip is absorbed by the porous mold, leaving the ceramic powder component of the slip in the cavity to form the shaped article. Once the shaped article is sufficiently dry, it is removed from the mold cavity. To facilitate removal, it is typically necessary to apply a mold release agent on the surfaces of the mold cavity.
Both hollow and solid articles can be formed by slip casting processes. Solid articles are formed by allowing the entire slip mass to dry within the cavity, while hollow articles require draining the excess slip from the cavity after sufficient time has passed, so as to form a suitably thick dry ceramic layer on the wall of the cavity. After slip casting, the shaped article conventionally undergoes a firing or sintering operation so as to fuse and/or react the ceramic powder, thus forming a strong and dense solid ceramic article.
Molds for slip casting preferably have controlled porosity so as to facilitate removal of the liquid from the slip by capillary action. Gypsum (CaSO.sub.4 .cndot.2H.sub.2 O) is widely used for making porous molds for slip casting processes. Through gypsum molds are suitable for general slip casting processes, they have pores which are not generally uniform, ranging in size between about 0.4 and about 5 micrometers, with the bulk of the pores being about 1 to 2 micrometers. In addition, gypsum molds are typically delicate and susceptible to wear, resulting in a shortened service life.
Gypsum is also known to dissolve in acids and ammonium (NH.sub.4) salts and, further, is soluble in water at about 0.241 grams per 100 ml. As a result, the cavity surface of a gypsum mold will eventually become deteriorated by the aqueous medium of the slip. In addition, when slips are stabilized in an acidic or basic solution for purposes of dispersing the ceramic particles within the aqueous solution, deterioration of gypsum molds has been found to be even more severe since the acid that accumulates on the mold surface during the casting operation reacts with the gypsum. Accordingly, an alternative mold material has been sought by the casting industry.
The use of porous ceramic molds has been previously suggested for use as investment casting molds. Ceramic molds are advantageous in that they are typically strong, have long service life and are substantially nonreactive with aqueous components used in slip compositions. However, the use of a porous ceramic mold for a slip casting process requires a different mold release agent due to the different composition, porosity and surface characteristics of ceramic molds, as compared to gypsum molds.
Generally, mold release agents must be compatible with the mold material and prevent the slip from adhering to the mold cavity. In addition, mold release agents must not adversely effect the final properties of a cast ceramic article, which may occur as a result of the slip being in direct contact with the mold release agent during the molding process. This is particularly true when submicron ceramic powders are intended to be used in the slip casting process. Ceramic articles formed from submicron ceramic powders are desirable in that they are more capable of being sintered to full density at relatively low temperatures as compared to ceramic articles formed from larger-sized particles. However, the submicron particles tend to be more adherent to the molds, thus requiring more stringent control over the materials, porosity and surface features of the mold release agent and mold.
As an example, undesirable flocculation of the slip may occur, wherein the ceramic particles tend to clump together, thereby resulting in an inferior non-uniform dispersion of the ceramic particles throughout the slip. In addition, during the subsequent sintering of the cast article, unwanted residue due to the mold release agent on the surface of the sintered article may produce undesirable changes in the microstructure of the sintered article and/or detrimentally affect the physical properties of the sintered object.
Conventionally, ammonium alginate (C.sub.6 H.sub.7 O.sub.6 .cndot.NH.sub.4) mold release agents have been used satisfactorily with gypsum molds. Ammonium alginate-type mold release agents are dissolved in water to form a solution. The solution is then poured into the mold cavity and allowed to sit for a duration which is sufficient to form a continuous film on the surface of the mold cavity, with a portion of the solution being adsorbed into the mold. The excess solution is then drained out of the mold cavity. The continuous thin film serves as a permeable barrier layer between the slip and the gypsum mold, so as to promote release of the cast article afterward while also allowing passage of the water within the slip through to the porous ceramic mold.
However, it has been determined that ammonium alginate-type mold release agents do not perform well with ceramic molds. This is primarily because the cast articles do not easily release from the mold cavity. The reason for this is believed to be that, while ammonium alginate-type mold release agents are compatible with gypsum molds, they are generally incompatible with ceramic molds due to a ceramic mold's different chemistry and pore structure. Other known mold release agents, such as silicone, graphite, oxides, talc, olive oil and spray starch, have also been determined to be unsuited or otherwise less than optimal for use with ceramic molds.
Thus, it would be desirable to provide a mold release agent which would permit the use of a porous ceramic mold in a slip casting process, wherein the mold release agent facilitates the removal of cast articles from the mold cavity, yet without promoting flocculation of the slip during the slip casting process or adversely affecting the surface characteristics or the sintering behavior of the cast article such as by leaving a residue on the cast article.